Re: 1950's freight rules

Since it's fun having an excuse to pull out some old timetables and special instructions, here's a few things I found from a quick search that might help.
The only rules that I have found are those governing engine placement, and these changed a bit over the years. Mike basically covered them. However there were potentially other rules that got covered by bulletin or general notice, and not in the special instructions.
From a 1953 timetable: No ng. trains over 70 cars. No doubleheading of K-36 and K-37's over the trestles, three engine trains up Cumbres put two on the point and one ahead of the caboose, trains over 35 cars from Antonito to Cumbres must cut the second engine into the train. Second engines between Chama and Durango must be on head end. Engines will not be placed behind ng. cabooses except in emergency and only engines smaller than K-27.
By 1960 when I first visited two engine Cumbres turns always (at least when I was there) had the helper cut in next ahead of the caboose, but I think in earlier years they were sometimes cut into the train.
Off the top of my head, later on the criteria for cutting engines in at Antonito was changed to tonnage, and doubleheading was prohibited downhill between Chama and Durango. There's probably more, but that's what I could find from a quick perusal.
With regard to steel cars on the headend, I'm not aware of any rule governing this. The empty tanks from Alamosa to Chama were almost always on the head end, but I believe this was a blocking issue not related to their steel underframe. It made it easy for the tanks to be cut off and shoved back to the loading rack upon arrival at Chama. Climbing Cumbres I have seen plenty of pictures of the loaded tanks behind wooden underframe cars, etc. And steel frame flats at the rear of a train.
I would guess that east bound trains were pretty much "mill run" (in whatever order was convenient). Westbound trains probably had a Chama block, a Farmington block, and any cars for Durango ahead of the caboose. But that's just a guess.
It was also common to see wooden cars with the draft gear pulled out.
Hope that helps.
JBW